1. Field of the Invention
The present invention relates to an image analysis device which is to be installed on a vehicle, and which detects objects that are likely to influence travel of the vehicle from an image of a field taken by an image-taking section.
2. Description of the Related Art
An example of such a device installed on a vehicle, which detects objects, such as pedestrians that may collide with the vehicle, from images of the surrounding environment taken by an image-taking section consisting of, for example, infrared cameras, and which provides such information to the driver of the vehicle, is disclosed, for example, in Japanese Unexamined Patent Application, First Publication No. 2001-6096.
In this device, the images of the surrounding environment are taken by a pair of stereo-infrared cameras that are disposed laterally, and a right-hand of the field taken by one of the infrared cameras disposed in the right undergoes a segmentation process in order to extract a first image of an object. Next, a seeking area, in which seeking of an image corresponding to the first image of the object is executed, is set in a gray scale image of a left-hand image of the field taken by the other of the infrared cameras disposed in the left, and correlation calculation processes are sequentially applied to the first image of the object and images of objects in the seeking area in order to extract another image of the object, i.e., a second image of the object, corresponding to the first image of the object. Moreover, a parallax Δd (the number of pixels) is calculated based on the positional difference between the geometric center of the first image of the object in the right-hand image of the field and the geometric center of the second image of the object in the left-hand image of the field, and then the distance “z” between the vehicle and the object is calculated.
Next, the correlation calculation process will be more specifically explained. The distance “z” to the object is calculated using the following equation (1) based on an object parallax Δd obtained by the pair of cameras:z=f×D/(Δd×p)  (1)where “f” is a focal length of the camera, D is the distance between the pair of stereo cameras (the length of a baseline), and “p” is the pitch between the pixels.
As shown in FIG. 10, the object parallax Δd is determined in such a manner that a reference pattern “Pat” (“Pat” is defined by Pat_dx×Pat_dy) of the right-hand image of the field is shifted one pixel by one pixel in the seeking area “Area” (“Area” is defined by Area_dx×Area_dy) of the left-hand image of the field so as to find a position of the reference pattern “Pat” which gives the highest degree of correlation.
In this process, a “SAD (Sum of Absolute Difference)” process is used to find a position cx of the reference pattern “Pat” where a correlation error “Error”, which is defined by the following equation (2), is minimized, as the position which gives the highest degree of correlation:Error(cx)=ΣiΣj|Pat(i)(j)−Area(i+cx)(j)|  (2),where 0≦cx<(Area_dx−Pat_dx+1), 0≦i<Pat_dx, and 0≦j<Pat_dy.
Accordingly, assuming that a set of calculations is required when the difference between “Pat” and “Area” corresponds to one pixel, the number of sets of calculations in the equation (2) is expressed by the following equation (3):Calc—N=(Pat—dx×Pat—dy)×(Area—dx−Pat—dx+1)  (3).
When a device, in which the correlation calculation process according to the above algorithm is executed, is used, a portion of the second image of the object, which gives the highest degree of correlation with the first image of the object that is extracted from the right-hand image of the field, can be accurately extracted from the seeking area in the second image of the object; however, the calculation time is substantially determined by the processing capacity of the associated hardware.
More specifically, when a detection, in which objects that are likely to influence travel of the vehicle are sought in the images taken by the image-taking section, is performed, because the objects whose images are being taken by the image-taking section move continuously as the vehicle runs, a computer having an extremely high-speed processing capacity is required in order to complete the correlation calculation processes between the first image of the object extracted from the right-hand image of the field and the seeking area set in the gray scale image of the left-hand image of the field within a period during which the objects are being taken by the image-taking section; however, such a high performance computer is quite expensive, consumes a considerable amount of electrical power, and is difficult to be installed on a vehicle in terms of space for installation.